Among methods for assay of an analyte in a test solution is one using a probe labeled with a chemiluminescent substance. This method is widely used as being capable of determining the amount of the analyte highly sensitively by measuring the quantity of chemiluminescence. This method is useful for a suitable amount of the analyte. In the presence of a large amount of the analyte (e.g. when many copies are produced by gene amplification such as polymerase chain reaction), however, the quantity of chemiluminescence by the method exceeds the determination limit of a measuring device, making accurate assay impossible. Such samples with results in excess of the upper assay limit have been determined again after dilution of the test solution. This procedure is very laborious. Samples amplified by gene amplification, in particular, can cause contamination of the amplified product as a result of dilution. Utmost care has been taken to avoid the contamination, further increasing labor. To broaden the range of assay without diluting the sample, there has been no choice but to wait for an improvement in the measuring device.
We, the inventors, have found that the foregoing problems with the determination of a sample beyond the assay limits can be solved by decreasing the quantity of chemiluminescence, without requiring a laborious operation such as the dilution of the sample, or an improvement in the measuring device. This finding has led us to accomplish this invention.